We are studying cell fusion in the epithelial cells of the nematode, Caenorhabditis elegans. We wish to elucidate the ternporospatial distribution of nascent fusion pores between the precursor cells of the syncytial hypodermis of developing embryos. Cell fusion events will be observed by 4-dimensional timelapse recording, using differential interference contrast as well as laser-scanning confocal or multiple-photon excitation imaging of fluorescent probes labeling the plasma membrane and cytoplasm. Using high-pressure preservation, individual embryos which have been observed live by light microscopy will be examined by transmission electron microscopy to ascertain the ultrastructure of actively fusing cell membranes.